Friction — Class 8 Science NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 8 Science chapter "Friction" — 8 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 8 Science chapter "Friction" — 8 important questions with detailed answers for CBSE board exam preparation…
Written & reviewed by the Syllab.in Academic Team (CBSE/NCERT subject experts) · Updated
NCERT Solutions for Class 8 Science — Friction. Step-by-step answers to all 8 textbook questions from this chapter, written for CBSE board preparation and free to use.
Friction — All 8 Questions Solved
Q1. Define friction. Why is it necessary?
Friction is the force that opposes relative motion between two surfaces in contact. It is necessary because: (1) It enables us to walk without slipping, (2) Vehicles can move and stop, (3) Prevents objects from sliding, (4) Helps us hold objects. Without friction, motion control would be impossible.
Q2. Name two ways to reduce friction between surfaces.
(1) Lubrication: Use oil or grease on moving parts to reduce friction. (2) Polishing: Make surfaces smoother to decrease friction. (3) Rolling: Replace sliding with rolling motion (wheels are more efficient). (4) Air cushion: Create a layer of air or fluid between surfaces.
Q3. What is static friction? Give an example.
Static friction is the friction that acts on an object at rest, preventing its motion. Example: A book on a table experiences static friction that prevents it from sliding. The friction force equals the applied force until the object starts moving.
Q4. Explain why sliding friction is less than static friction.
Static friction acts to prevent motion and can increase up to a maximum limit. Kinetic (sliding) friction acts on a moving object and is always less than maximum static friction. Once motion begins, intermolecular bonds are broken, reducing resistance. This is why it's harder to start moving an object than to keep it moving.
Q5. How does friction depend on the nature of surfaces?
Friction depends on the roughness of surfaces: (1) Rough surfaces have high friction, (2) Smooth surfaces have low friction. The microscopic irregularities on surfaces interlock, creating friction. Polished surfaces have fewer irregularities, resulting in less friction. Material type also affects friction coefficient.
Q6. Is friction always harmful? Explain with examples.
No, friction is not always harmful. Beneficial friction: Walking on ground, writing with pencil, braking vehicles, tyres gripping roads. Harmful friction: Wear of machinery parts, energy loss in engines, rubbing causing heat. Friction is essential for life; we manage it rather than eliminate it.
Q7. Why is the coefficient of friction higher for rubber tyres on dry roads than on wet roads?
On dry roads, rubber tyres grip the road surface directly, creating high friction. On wet roads, a thin water layer forms between the tyre and road, reducing contact area and friction. This causes skidding and loss of traction. Water acts as a lubricant, decreasing the coefficient of friction.
Q8. What is rolling friction? Why is it less than sliding friction?
Rolling friction is the friction that opposes rolling motion. It is less than sliding friction because: (1) In rolling, only the contact point changes, no continuous sliding, (2) Less surface area is engaged, (3) Deformation is minimal. Wheels are efficient because rolling friction is much lower than sliding friction between surfaces.
Friction — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. Which force always opposes the motion of an object?
- A. Gravitational force
- B. Muscular force
- C. Frictional force
- D. Magnetic force
Answer: C. Frictional force — Frictional force is defined as the force that opposes the relative motion between two surfaces in contact.
MCQ 2. What is the main cause of friction between two surfaces?
- A. Attraction between molecules
- B. Interlocking of irregularities
- C. Smoothness of surfaces
- D. Presence of air
Answer: B. Interlocking of irregularities — Friction is caused by the interlocking of tiny irregularities present on the surfaces that are in contact with each other. Even seemingly smooth surfaces have microscopic irregularities.
MCQ 3. Which type of friction is generally the greatest?
- A. Sliding friction
- B. Rolling friction
- C. Static friction
- D. Fluid friction
Answer: C. Static friction — Static friction is the friction that must be overcome to start an object moving from rest, and it is generally greater than sliding or rolling friction.
MCQ 4. What is the primary purpose of using lubricants like oil or grease in machines?
- A. To increase weight
- B. To increase friction
- C. To reduce wear and tear
- D. To enhance magnetic properties
Answer: C. To reduce wear and tear — Lubricants form a thin layer between moving parts, reducing the direct contact and interlocking of irregularities, thereby decreasing friction and minimizing wear and tear.
MCQ 5. Which of the following activities is NOT possible without friction?
- A. Swimming
- B. Flying a kite
- C. Walking on the ground
- D. Riding a bicycle downhill
Answer: C. Walking on the ground — When we walk, friction between our feet and the ground allows us to push backward, and the ground pushes us forward. Without friction, our feet would simply slip.
MCQ 6. Why do car tires have treads (grooves)?
- A. To make them look good
- B. To reduce their weight
- C. To increase friction for better grip
- D. To make them last longer
Answer: C. To increase friction for better grip — Treads on tires increase the rough surface area and provide channels for water to escape, enhancing friction and providing better grip on the road, especially in wet conditions.
MCQ 7. The friction experienced by an object when it is rolling over a surface is called:
- A. Static friction
- B. Sliding friction
- C. Rolling friction
- D. Fluid friction
Answer: C. Rolling friction — Rolling friction is the force resisting the motion when a body rolls over a surface. It is generally much smaller than static or sliding friction.
MCQ 8. Which of these statements about friction is TRUE?
- A. Friction always helps motion
- B. Friction always produces cold
- C. Friction causes wear and tear
- D. Friction is independent of the nature of surfaces
Answer: C. Friction causes wear and tear — Friction opposes motion and converts kinetic energy into heat, leading to wear and tear of surfaces in contact.
MCQ 9. Why are the soles of sports shoes often designed with patterns and rough surfaces?
- A. To make them look stylish
- B. To increase their lifespan
- C. To decrease the cost of production
- D. To provide better grip and prevent slipping
Answer: D. To provide better grip and prevent slipping — The patterns and rough surfaces on sports shoe soles increase the friction between the shoes and the ground, offering better grip and reducing the chances of slipping during activities.
MCQ 10. Which of the following substances can be used as a lubricant?
- A. Water
- B. Sand
- C. Graphite powder
- D. Salt
Answer: C. Graphite powder — Graphite powder is a solid lubricant commonly used in machine parts where oil cannot be used, as it reduces friction between surfaces. Water and sand would typically increase friction.
MCQ 11. When a stationary object just begins to move, the friction acting on it is called:
- A. Sliding friction
- B. Rolling friction
- C. Static friction
- D. Kinetic friction
Answer: C. Static friction — Static friction is the force that opposes the initiation of motion of a stationary object. Once the object starts moving, it becomes sliding friction.
MCQ 12. What happens to friction if the surfaces in contact are pressed harder against each other?
- A. It decreases
- B. It increases
- C. It remains the same
- D. It becomes zero
Answer: B. It increases — When surfaces are pressed harder, their irregularities interlock more tightly, leading to an increase in the frictional force between them.
MCQ 13. Why do carrom board players sprinkle powder on the board?
- A. To make the board shiny
- B. To increase friction for better grip
- C. To reduce friction for smoother play
- D. To clean the board
Answer: C. To reduce friction for smoother play — Sprinkling powder on a carrom board fills the microscopic irregularities on its surface, making it smoother and significantly reducing friction, allowing the pieces to glide easily.
MCQ 14. Which of the following methods helps to reduce friction?
- A. Making surfaces rough
- B. Using treads on tires
- C. Streamlining objects
- D. Increasing the contact area
Answer: C. Streamlining objects — Streamlining objects (like cars, boats, and aeroplanes) reduces fluid friction (air or water resistance) by giving them a shape that allows them to cut through the fluid more easily.
MCQ 15. Friction is considered a 'necessary evil' because:
- A. It always helps us
- B. It always harms us
- C. It has both advantages and disadvantages
- D. It is only present in machines
Answer: C. It has both advantages and disadvantages — Friction is necessary for many daily activities (like walking, braking) but also causes disadvantages (like wear and tear, energy loss), hence it's called a 'necessary evil'.
MCQ 16. Which type of friction is utilized when you apply brakes to a bicycle?
- A. Static friction
- B. Rolling friction
- C. Sliding friction
- D. Fluid friction
Answer: C. Sliding friction — When brakes are applied, the brake pads press against the wheel rim, causing a sliding motion and generating sliding friction, which slows down or stops the bicycle.
MCQ 17. The amount of friction depends on:
- A. The size of the object
- B. The colour of the object
- C. The nature of the surfaces in contact
- D. The speed of the object (at low speeds)
Answer: C. The nature of the surfaces in contact — Friction primarily depends on the roughness or smoothness (nature) of the surfaces that are in contact. Rougher surfaces generally exert more friction.
MCQ 18. Why do Kabbadi players rub their hands with soil?
- A. To clean their hands
- B. To show tradition
- C. To increase friction for better grip
- D. To make their hands heavier
Answer: C. To increase friction for better grip — Rubbing hands with soil makes them rougher, thereby increasing friction. This allows Kabbadi players to get a better grip on their opponents and prevents slipping.
MCQ 19. If you push a box on a rough floor, and then on a polished marble floor with the same force, what will be observed?
- A. The box will move faster on the rough floor
- B. The box will move slower on the polished floor
- C. The box will move faster on the polished floor
- D. The box will not move on either floor
Answer: C. The box will move faster on the polished floor — A polished marble floor has less friction than a rough floor. With less friction opposing the push, the box will accelerate and move faster on the polished floor.
MCQ 20. What happens to energy lost due to friction in machines?
- A. It is completely destroyed
- B. It is converted into electrical energy
- C. It is converted into heat energy
- D. It is stored as potential energy
Answer: C. It is converted into heat energy — Friction converts some of the kinetic energy of moving parts into heat energy, leading to heating up of machines and energy wastage.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.